Activity: Calculate the velocity and acceleration of a
coronal mass ejection (CME) based on its position in a series of
images from the Large-Angle Spectrometric Coronograph (LASCO)
instrument on SOHO.

Background: An important part of space weather research is
to measure the velocity of CMEs and their acceleration as they
leave the Sun. This is done by tracing features in the CME and
measuring their positions at different times. In the sequence of
images shown on the right, you can see a CME erupting from the Sun
on the right side of the coronagraph disk. The white circle shows
the size and location of the Sun. The black disk is the occulting
disk that blocks the surface of the Sun and the inner corona. The
lines along the bottom of the image mark off units of the Sunís
diameter.

Procedure: Select a feature of the CME that you can see in
all five images--for instance, the outermost extent of the cloud, or
the inner edge. Measure its position in each image. Your
measurements can be converted to kilometers using a simple ratio:

Using the distance from the Sun and the time (listed on each
image), you can calculate the average velocity. Velocity is
defined as the rate of change of position. Using the changes in
position and time, the velocity for the period can be calculated
using the following equation: v = (s2 - s1) / (t2 - t1), where s2
is the position at time, t2; s1 is the position at time, t1. The
acceleration equals the change in velocity over time; that is, a =
(v2 - v1) / (t2 - t1), where v2 is the velocity at time t2; v1 is
the velocity at time t1. You can record your results in a table.

Further Questions and Activities

Select another feature, trace it, and calculate the velocity
and acceleration. Is it different from the velocity and
acceleration of the other feature you measured? Scientists often
look at a number of points in the CME to get an overall idea of
what is happening.

How does the size of the CME change with time? What kind of
forces might be acting on the CME? How would these account for
your data?

Credit: This activity is based on "Sun-Centerd
Physics,"a set of lesson plans developed by Linda
Knisely.